A tunnel boring machine (“TBM”) is a tunnel excavation apparatus for constructing a tunnel through soil and rock strata. Modern TBMs produce a smooth circular tunnel wall, typically with minimal collateral disturbance. A breakthrough that made TBMs efficient and reliable was the invention of the rotatable cutter assembly, developed by James S. Robbins, which is configured to be mounted on a rotating cutterhead. Previous TBMs used rigidly-mounted spikes on a rotating head that were positioned to engage and bore into the ground. However, the rigidly-mounted spikes would often break, resulting in frequent and expensive downtime for the TBM. Robbins discovered that by replacing the rigid spikes with rotatable cutter assemblies the reliability of the TBM was greatly improved. Since then, successful modern TBMs have rotatable cutter assemblies.
Modern TBMs employ rotating cutterheads with rotatable disc cutter assemblies that are mounted to the cutterhead. The cutterhead is urged against the target surface with a large thrust force, for example, using a plurality of hydraulic cylinders or other mechanical actuators, such that the cutter assemblies engage the surface. As the cutterhead rotates, the rotatable cutter assemblies fracture, crush, and/or loosen materials which are then transported away by the TBM as the TBM progresses to bore the tunnel.
FIG. 1 is a cross-sectional view of a prior art cutter assembly 80 for a tunnel boring machine (TBM not shown). See, for example, U.S. Pat. No. 5,904,211, to Friant et al., which is hereby incorporated by reference in its entirety. The cutter assembly 80 includes a shaft 81 that is configured to be fixedly attached to the TBM rotating cutterhead. An annular cutter ring 82 (sometimes called a cutter disc) is attached to a hub 83 with an interference fit between the hub 83 and the cutter ring 82, and positioned with retainer ring 84, to form a ring assembly 85. The ring assembly 85 is rotatably mounted to the shaft 81 with a pair of bearing assemblies, in this embodiment comprising an inner bearing race 87, an outer bearing race 88, and a plurality of tapered roller bearings 89. End retainers 90, 91 are disposed on either side of the bearing assemblies. During operation, the ring assembly 85 is rotatable about the shaft 81, and the end retainers 90, 91 are fixed to the shaft 81.
A rotary seal group 92 is provided at the interface between each of the end retainers 90, 91 and the ring assembly 85. The rotary seal groups 92 are mechanical face seals, also referred to as duo cone seals. The mechanical face seals were developed for protecting equipment working in the most adverse conditions, and comprise a pair of annular metal seal rings 93, and a pair of elastic toric members 94 (e.g., O-rings). The outer metal seal ring 93 engages the associated end retainer 90 or 91 through a toric member 94 and is fixed, and the associated inner metal seal ring 93 engages the ring assembly 85 through a toric member 94 and rotates. The two associated metal seal rings 93 abut to form a moving seal interface there between. Typically the available interior volume between the end retainers 90, 91 is filled with a lubricant, e.g., oil or grease. The rotary seal groups 92 provide a seal to prevent the incursion of dirt that could damage or destroy the bearing assemblies.
An alternative approach to a TBM cutter assembly construction is disclosed in U.S. Pat. No. 7,017,683, to Narvestad. Narvestad discloses a divided cutter ring mounted on a tubular cutter body having a slanted surface on one side that receives a correspondingly angled base of the cutter ring. A clamping ring with a slanted inner surface is threadably tightened onto the tubular cutter body opposite the cutter ring to capture the wide base of the cutter ring in a dovetail arrangement. The cutter ring construction disclosed by Narvestad includes a relatively large and massive base portion, so the cutter ring remains relatively costly.
There exists a need for improved cutter assemblies for tunnel boring machines.